Since the last Google PageRank update was done April 2, 2010. The next PageRank update will be scheduled by Google somewhere around the first week of July 2010, as every update generally is displayed for public viewing every 4 months, it is very probable that it will be in July.

There are no rules about this, if Google wants to update the public value of PageRank every 1 or 6 months, it is just up to them.

In reality, there is a private value of PR only visible to Google that changes everyday.